A typical electrophotographic printing device exposes a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive member are discharged to record thereon an electrostatic latent image corresponding to the original image document. A developer mix of carrier granules and toner particles are moved into contact with the latent image recorded on the photoconductive member. Toner particles are attracted electrostatically from the carrier granules to the latent image. In this manner a powdered image is formed on the photoconductive member. Thereafter, the powder image is transferred to a sheet of support material. Subsequent to the transfer, the sheet of support material passes through a fusing device which permanently affixes the powder image thereto.
In photographic printing devices, tone gradations are difficult to form. To minimize this difficulty, screening methods are utilized. Generally, a screening technique produces the effect of tone gradations by variations in dot or line size. In the highlight zones, the dots or lines are small. These dots or lines increase in size through the intermediate shades until the dots or lines merge together in the shallow region. At the highlight end of the tone scale, there may be complete whiteness, while at the shadow end of the tone scale, there may be nearly solid black.
The illumination of an image point is in proportion to the cosine to the fourth power of the angle between the illumination point and the image point. The illumination on a photoconductive surface will fall off quite rapidly as the angle increases. Various techniques have been devised to compensate for this effect. Typically, a sheet of opaque material having a butterfly slit formed therein is employed. The area of the slit is inversely proportionally to the illumination profile. In an exposure system of this type, the original document is positioned on a flat transparent platen. Scan lamps and lenses move across the original document in synchronism with the rotation of the photoconductive drum. Successive incremental areas of the original document are scanned forming a flowing light image which is projected through the slit.
A well known characteristic of such slit exposure systems where the original document is positioned on a flat platen and the light image passes through the slit onto a curved photoconductive member, is image smearing. Image smearing occurs even if the scan and drum velocity are properly synchronized. The loci of exposure points on the drum corresponding to a single point of the original document are defined by the intersection of a plane and a cylinder. A plane is defined by a point and a line to the drum axis and containing any image point on the photoconductive drum. During a slit scan, the image point does not remain stationary on the drum, but rather suffers both lateral and longitudinal translations. Such image motion causes loss of resolution. If a screen member having a plurality of equal spaced opaque lines were placed near the photoconductive surface wherein the lines were aligned parallel to the drum circumference, the lateral image would smear the modulation produced by the screen member. In extreme cases, the modulation could be destroyed near the edges of the drum where smearing is a maximum. For typical drum radii, lens focal lengths and slit widths, this lateral smearing is significant.
Various techniques have been proposed to minimize the smearing as a result of the modulation produced by a screen member for producing half-tone image gradations. Such techniques have utilized arcuate screen members positioned adjacent to the photoconductive drum. One such screen member has a curvature substantially equal to the curvature to the photoconductive member where the center of curvature is substantially in coincidence with the center of curvature of the photoconductive member. Another such proposed system utilizes an arcuate screen member with the plane of curvature of the screen member being substantially normal to the plane of curvature of the photoconductive member. On such screen members, opaque lines are utilized. However, such screen members are difficult to fabricate as well as result in increased costs of such electrophotographic printing devices.
A need has thus arisen for an improved optical system for an electrophotographic printing device such that modulations produced by a screen will not be smeared.